As is known, current data-storage systems that exploit a technology based upon magnetism, such as, for example, computer hard disks, suffer from important limitations in regards to data-storage capacity, read/write speed, and dimensions.
In recent years, there have consequently been proposed alternative data-storage systems, in particular based upon techniques of silicon micromachining, with the purpose of achieving data-storage densities and read/write speeds that cannot be achieved with traditional techniques.
For example, the data-storage system proposed by IBM and referred to as “Millipede” (see in this regard “The “Millipede”—Nanotechnology Entering Data Storage”, P. Vettiger et al., IEEE Trans. on Nanotechnology, Vol. 1, No. 1, March 2002) exploits a type of technology based upon silicon nanometric read/write heads, similar to the ones used in atomic-force microscopes (AFMs) or in scanning tunnelling microscopes (STMs) to obtain images at an atomic scale. For a better understanding, reference may be made to FIG. 1, which shows a general diagram of the “Millipede” system.
As illustrated in FIG. 1, a mass storage device 1 according to the “Millipede” system is made up by a two-dimensional array 2 of cantilever elements 3, of silicon, which are obtained exploiting micromachining techniques and are fixed to a common substrate 4, also of silicon. Each of the cantilever elements 3 functions as a support for a respective read/write (R/W) head 6 formed at the end of the respective cantilever element 3.
A polymer film 5, for example of polymethylmethacrylate (PMMA) and operating as a data-storage material, extends underneath the two-dimensional array 2 and has the thickness of some tens of nanometers.
Each individual R/W head 6 can be driven for reading or writing via an addressing technique similar to the one commonly used in DRAMs, hence through two multiplexers 10 and 11, which respectively select the rows and columns of the two-dimensional array 2.
The polymer film 5 is located on a mobile platform 12, which is moved in the directions x, y and z by an actuating system (not shown) comprising miniaturized windings and permanent magnets.
Each R/W head 6 acts within its own restricted data-storage area, typically of the order of 100 μm2, so that, for example, in a 32×32 array, 1024 R/W heads 6 are present.
Each cantilever element 3 stores data through the respective R/W head 6, by forming, in the polymer film 5, indentations 14 (shown only schematically in FIG. 1) having a width and a space between them of some tens of nanometers.
The presence or absence of an indentation 14 encodes a datum to be stored in a binary way (for example, the presence of an indentation can represent a “1”, whilst the absence of an indentation can represent a “0”).
During writing, the indentations 14 are created by applying a local force on the polymer film 5 via the R/W heads 6 and, at the same time, by locally heating the polymer film 5 at high temperature (approximately 400° C.). Heating is performed by a heater element of a resistive type, here made of a silicon monocrystal with a low doping level, arranged at the R/W head 6 and the passing of an electric current. When the R/W head 6 has reached the desired temperature, it is brought into contact with the polymer film 5, which is locally softened by the heat; consequently, the R/W head 6 penetrates within the polymer film 5, generating the indentation 14.
Reading is carried out using the heater element as a temperature sensor, exploiting the variation in its resistance as a function of the temperature.
In particular, the resistance of the heater element increases with the temperature in a non-linear way starting from room temperature up to a peak value between 500° C. and 700° C. and which depends upon the concentration of dopant in the heater element.
During reading, the heater element is heated at a constant temperature of approximately 350° C., a temperature which does not cause softening of the polymer film 5 but is sufficient to create a temperature gradient with respect to the polymer film 5, as is necessary to read the data. In fact, heat is transferred between the R/W head 6 and the polymer film 5, through the air, and the heat transfer becomes more efficient when the distance between the two elements decreases, i.e., when the R/W head 6 moves inside an indentation 14. Consequently, when the R/W head 6 is in the indentation 14, the temperature of the heater element decreases, and consequently its resistance is reduced.
The variation in resistance of the heater element can thus be used for reading the stored data.
Stored data erasing can occur according to two different modes, as described hereinafter.
First, an entire data block can be erased by heating the polymer film 5 at a high temperature for a few seconds. After cooling, the surface of the polymer film 5 becomes uniform again.
Second, an individual data bit can be erased by bringing the R/W head 6 in write mode to a point adjacent to the indentation 14 to be erased. In fact, in this point, polymer molecules are concentrated on account of the previous formation of the indentation 14, which, on account of the force exerted by the R/W head 6, are now forced to redistribute uniformly, thus causing an effect of evening-out which removes the stored information.
The described device enables data-storage capacities of the order of the terabits to be obtained in an extremely small space (a few tens of square millimeters).
The use of a temperature sensor of a resistive type during reading of the stored data is, however, disadvantageous in so far as the resistance of the sensor is affected not only by the data to be read, but also by the geometrical dimensions of the sensor, according to the formula:R=ρL/A where ρ is the resistivity of the sensor made of doped silicon, L is its length, and A is the surface of its cross section.
In planar silicon processes, the geometrical dimensions of the individual elements are hard to reproduce in a precise and constant way. For this reason, in a storage system of the type described previously, comprising even several thousands of read/write heads, it is reasonable to expect a certain spread in the geometrical dimensions of the various heads and hence in their resistance values.
Consequently, at worst, a difference in the value of the resistance of a R/W head 6 due to its different geometrical configuration can be erroneously associated with reading a different data bit, thus causing errors in the detection of the stored data. It must, in fact, be taken into account that the relative resistance differences (ΔR/R) to be detected, that indicate the read data bits, are of the order of 10−4, and hence not much greater than the differences due to production errors.